United 93 is a limited release in selected theaters.
I do not know why this stratagy was chosen but F93 is packing more people in per theater than #1 or #2
per-theater
#1 - 3,639 theaters - $1,240 each
#2 - 2,o4o '' - $1,970 each
#3(F93) 1,795 '' - $2,050 each
I don't believe it was a matter of "choosing a strategy." It's really hard to get a movie into the mainstream theaters. Hundreds of independent movies are made each year, and very few of them hit the big screen.
The big studios "own" the distribution system.